When primary and secondary pipes with different diameters are obliquely crossed and butt-jointed at any angle in the butt-joint process of reducing pipe fittings, the contact surface between the primary pipe and the secondary pipe is an irregular curved surface, leading to high difficulty in onsite mapping and processing; an experience-based layout method is typically employed in practical production, however, multiple processing needs to be carried out before progressive attached forming is reached, which is accordingly against fast production.